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SELF and NON-SELF
 

The immune system has the ability to distinguish between self and non self. Every body cell carries distinctive molecules that identify it as self. This ability to distinguish between self and non-self is necessary to protect the organism from invading pathogens and to eliminate modified or altered cells. Immune cells and other body cells coexist in a state known as self-tolerance. When immune defenders encounter cells or organisms carrying molecules that say "foreign," the immune defenders act to eliminate the intruders.

In some cases the immune response can be directed toward self tissues resulting in autoimmune disease, which is the failure of an organism to recognize its own parts as self, creating an immune response against its own cells and tissues.

Pathogenic microbes or other foreign bodies that trigger an immune response are called antigens. Tissues or cells from another individual - except an identical twin whose cells carry identical self-markers - also act as antigens and because the immune system recognizes transplanted tissues as foreign, it rejects them.

Antibodies are Y-shaped proteins that read surface proteins of every cell they encounter. They stick to foreign cells, marking the antigen for destruction by macrophages and other leukocytes (white blood cells). T-cells are also immunological markers, which bind to foreign proteins, triggering events that lead to their destruction.

In the image below, the infected blood cell is considered foreign because of the antigens that attach themselves to the receptor molecules. The macrophage identifies this cell as being foreign, or infectious and engulfs it, thus destroying it.

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